The invention relates to a glass body, especially in the tempered state, in the form of a glass panel or glass pane, which is suitable for the production of highly heat-stable glazings which meet both fire retardant and safety glass requirements. Although the invention is described in terms of German requirements, the invention is also intended to conform to U.S. standards and requirements, e.g., those of the Underwriters Laboratories.
Fire retardant glazings including frames and hardware must conform to fire resistance classes in terms of retarding the passage of fire and smoke. In Germany, fire retardant glazings are classified into fire resistance classes G30, G60, G90 and G120. Installation in the frame results in the glass panel or panel being covered at its edge by the lip of the frame which retains the glass. In the event of an outbreak of fires, there arises between the covered edge of the pane and the exposed center of the pane a temperature difference which, depending on the depth and type of edge covering, can be between 200 and 350 K. The fire resistance period in a standard fire to DIN 4102 depends in the first instance on the softening temperature (EW), pane size, pane thickness and the edge cover, and on absorption, reflection and thermal conductivity of the glass. The wider the edge cover, the longer the glazing withstands being pulled out of the frame. The softening temperature of a glass is given in accordance with DIN 52312, Part 3, as the temperature at which the glass has a viscosity of 10.sup.7.6 dPas, approximately the same viscosity at which glassblowers form glassware.
In the case of glazings in buildings, the glazings must frequently meet a number of requirements simultaneously. Fire retardant glazings which are installed, for example, in doors, must ensure the safety of users in daily use, in addition to affording fire protection. Thus, glazings may not break easily under mechanical loading. If, for example, in spite of this breakage resistance, an impact by a human body against the glazing results in pane breakage, there should be formed only small, blunt-edged fragments as the glazing breaks (DIN 18361, Section 2.3.6.3).
In practice three types of monolithic glasses are currently used for fire retardant glazings. The most frequently used is wired lime-soda glass. In the event of an outbreak of fire, the wired glass cracks after only a few minutes. The pieces of glass are, however, held together by the embedded spot-welded wire mesh. Wired glasses are able to resist fire for up to 60 minutes in specific designs having relatively small pane dimensions. Under mechanical loading, however, wired glasses break more easily than normal glass panes of the same thickness because the glass exhibits fine internal flaws which are due to the wire mesh. Mechanical breakage may cause serious injury as a result of barb formation. A further disadvantage of wired glasses is their less attractive appearance.
Thermally highly-tempered lime-soda glasses (for example float glass) are also utilized as fire retardant glazings. The high coefficient of thermal expansion of these glasses means that such glass panes possess, even at a high compressive stress of 120N/mm.sup.2 --irrespective of pane thickness--a temperature difference resistance (TUF), between the cold edge of the pane and the hot center of only about 200 to 220 K. The temperature difference resistance characterizes the property of a pane to withstand the difference in temperature between the hot center of the pane and the cold edge. The parameter for the TUF is given as the temperature difference in degrees Kelvin between the maximum temperature of the hot pane surface in the central zone of the pane and the temperature of the (covered) cold edge of the pane, exceeding which as a rule results in stress fracture. The temperature difference resistance (TUF) is determined by the following standard method of measurement: panels (approx. 25.times.25 cm.sup.2) are heated in defined manner in the central zone of the panel (area approx. 254 cm.sup.2), with a 2 cm wide edge of the pane being held at room temperature. The ratio of the cold surface area to the hot surface area which is thus adjusted is selected such that the maximum permitted temperature difference determined by the standard measuring method is directly translatable to the majority of installation situations in conventional practice. The TUF is given as that temperature difference between the hot center of the pane and the cold edge, at which 5 or less percent of samples fail as a result of fracture.
So that in the event of fire the TUF, which in the case of lime-soda glasses is very low, for a fire protection glazing not to be exceeded, only a narrow retaining lip equivalent to a small rebated glass depth of 10 mm or less, is permitted, leading to more rapid heating of the edge of the pane. At approximately 780.degree. C., the softening temperature of lime-soda glass (float glass) is also relatively low, so that fire retardant glazings of thermally tempered lime-soda glass are able to resist fire for 30 minutes only in specific frame systems and at certain pane sizes and thicknesses, thus conforming only to the lowest fire resistance class, G30. The safety glass requirements (safe break) are, however, met by these glasses.
The third commonly used fire protection glass is a thermally tempered borosilicate glass having a low coefficient of thermal expansion .alpha..sub.20/300 of 3.3.times.10.sup.-6 K.sup.-1. The maximum compressive stress which can be achieved in this glass is about 50-60N/mm.sup.2 using a conventional air tempering plant which can achieve a heat transmission coefficient of approx. 300 Watt per (m.sup.2 .times.K). Despite this low compressive stress, the glasses nevertheless possess a temperature difference resistance in excess of 400 K as a result of the low specific thermal stress .psi. of 0.25. The specific thermal stress .psi. is calculated using the equation .psi.=.alpha..times.E/(1-.mu.), where E denotes the elastic modulus N/mm.sup.2 and .mu. the Poisson constant. These fire retardant glazings resist the heating-up process at the beginning of a fire even with a 20 to 30 mm edge cover. The higher softening temperature of 820.degree. C. enables these fire protection glazings, depending on design, to resist fire for 90-120 minutes. Although the low temper of 50-60N/mm.sup.2 is adequate to compensate for thermal stresses during the heating-up phase of a fire, however, it is insufficient to allow the glazing to disintegrate into fine fragments required by DIN 1249, Part 12 in the event of mechanical breakage. Consequently, these glazings cannot be used for all applications.
It is also known that chilling glasses by immersion in oil-coated water can achieve a heat transmission which is higher by a factor of about 10 than that achieved by air chilling. This makes it fundamentally possible to generate a compressive stress of approx. 100N/mm.sup.2 in a 5 mm-thick pane of borosilicate glass having a thermal expansion of only 3.3.times.10.sup.-6 K.sup.-1. The temperature difference resistance is in this way increased to some 600 K, and the glazing disintegrates into fine fragments in the event of mechanical breakage.
Although this increased temperature difference resistance would allow a wider edge cover, an edge cover wider than 30 mm does not afford appreciable advantages in terms of the fire resistance period and is therefore not used in practice. Tempering glasses by chilling in oil-coated water has considerable disadvantages in comparison with air chilling, and these have hitherto largely prevented its use in producing fire safety glasses. On the one hand, the process is technically more demanding and considerably more cost-intensive than air chilling in a conventional air tempering plant, and on the other, chilling in oil-coated water requires the panes to be heated up and chilled vertically, suspended on nippers. The impressions left by the nippers are, however, always a particular weak point from an aesthetic viewpoint in the case of glazings.